Water-soluble polymers are long known to be very useful in both agricultural and industrial applications. Water-soluble linear polyacrylamides, for example, are useful because of their superior properties of high solubility and low use rates, among others. By far, the major use for linear polyacrylamides is in the treatment of water, especially wastewater.
In wastewater treatment and uses like canal sealing, polyacrylamide causes flocculation or agglomeration of fine particles. Flocculation, where relatively light particles are attracted together to form heavier particles, causes the particles to sink rather than float, clarifying the water. In agricultural sprays, the ability of these polymers to attract and retain water and add viscosity is utilized to enhance the effectiveness of the sprays, in addition to other purposes.
These linear polyacrylamides are commonly available in three commercial forms. First, as a water-dispersible solid, polyacrylamides may be used in applications such as hydroseeding and canal scaling. This form slowly dissolves in water, but has a tendency to agglomerate when added too quickly or all-at-once to water. When this happens, the lumps that are formed take hours, days or even weeks to dissolve in water. This slow dissolution property is advantageous in some applications, but is a highly undesirable trait in situations that require quick dispersion of the polymer.
Second, water-dispersed polymers are used that have the distinct advantage of already being dissolved. Examples of water-dispersed polymers may be found in agricultural spray deposition aids, among others. However, not much polymer can be dissolved in water-only about two percent (2%), before the solution becomes too viscous to be handled easily.
Traditionally, the third historical form is a water-in-oil (W/O) emulsion. This involves utilizing a polyacrylamide where the polyacrylamide chains are contained in small droplets of water that are dispersed in oil by using emulsifiers to help make the two phases mix. Emulsions are droplets or “bubbles” of liquid, known to those practiced in the art as “micelles,” suspended in another liquid with which the first liquid will not mix. The micelles are often called the “discontinuous phase” and the suspending liquid is called the “continuous phase.”
In the case of polyacrylamide emulsions, the polyacrylamide polymer is dissolved in the discontinuous phase, in this case, the water phase, while the continuous phase is oil. This is known as a water-in-oil (W/O) emulsion or a reverse emulsion. This type of emulsion keeps the polymer in small packets of water, which burst open when the emulsion comes into contact with water. Polyacrylamide-based W/O emulsions disperse well in water with vigorous stirring, and are used prevalently in water treatment.
Such emulsions are also used, among other things, in pesticide tank mixtures to aid in preventing drift and increasing deposition on target species. The problems with W/O emulsions are that they form solid lumps and other forms when the emulsion is added to water with little or no agitation or if the water-to-emulsion ratio is too low. Also, emulsions are inherently unstable and will eventually break or separate into oil and water layers. The oil rises as a layer, and the water layer sinks. Since the polymer chains are now free to combine, because they are not separated by the oil “walls” (that is the oil and water separation or dividing line), they combine to form large lumps.
The polyacrylamide polymer itself comes in several types, defined by electrical charge of the polymer chain. The polyacrylamide polymer may be nonionic, anionic or cationic. The cationic form is commonly used in water treatment. In the agricultural applications, the cationic, or positively charged polymer, is rarely used, as it has a deleterious effect on aquatic wildlife. The nonionic or uncharged form is a reaction product of pure acrylamide, forming an uncharged, but water-soluble polymer that is quite inert in the environment.
Acrylamide is co-reacted with other monomers to form the cationic or anionic forms. To form the anionic polymer, acrylamide is most often reacted with an acrylate monomer that is further reacted so that it becomes negatively charged. The nonionic and anionic polymers have different properties. At lower levels in water, the anionic polymers build properties, such as viscosity, faster. Anionic polyacrylamide polymers are compatible with other charged molecules, such as are contained in fertilizers. However, they can react undesirably with certain other charged molecules. Thus, nonionic polyacrylamides are used in situations where the anionics are incompatible with other molecules.
The amount of charge is measured as a percent of the comonomer added. Thus, a polyacrylamide that is 30% acrylate and 70% acrylamide is called a 30 percent-charged polymer. This percentage may be expressed as weight or mole percent, depending on the manufacturer. Typically, if the polymer is a combination of the two monomers, the acrylic acid portion is reacted with base to form the acid salt. The polymer is then considered to be charged.
Microemulsions are a very recent, commercially available development. A microemulsion is a special type of emulsion that has the same basic structure as traditional emulsions, except that the droplets are smaller. Smaller droplets, by virtue of the solution physics involved, are very stable and the droplets do not combine or separate in solutions as traditional emulsions do. Microemulsions are also virtually clear, while sometimes having only a slight haze, as opposed to standard emulsions which are typically milky white.
Polyacrylamide microemulsions have their own disadvantages, however. The prevalent disadvantage of a polyacrylamide microemulsion is that if it is combined with water or aqueous solutions, the polyacrylamide microemulsion will tend to form a skin at the surface that drastically reduces water diffusion, such as the diffusion of oil and/or emulsifier combination into the water phase. This is due to the fact that there are very many small aqueous droplets near the surface of the emulsion. When the small aqueous droplets are combined with water, water diffuses quickly across the discontinuous phase and swells the micelles nearest the surface. The micelles swell, combine, burst and rupture, in that order.
This almost instantaneous bursting of many of the droplets entangles the polymer on the surface of the microemulsion and forms a barrier, which, in turn, slows diffusion of water further into the microemulsion and dispersion of the rest of the polymer. This phenomenon, sometimes known as “skin” or “skinning,” causes the same problems that traditional emulsions have in terms of dispersion and clean out.
Observers of microemulsions may actually observe that they are clear and therefore question the ability of the product to do the job intended or observe the presence, in this case, of polymer until the product is added to water, which causes the characteristic milky appearance and slimy feel of polyacrylamide emulsion added to water appear.
While each of the polymers and the delivery systems has distinct advantages, certain applications create great disadvantages for all polymers. For example, in agricultural fields that are watered using pivot irrigation, the polymer polyacrylamide is known to have been tested and shown to be effective at reducing the need for water. However, handling of the traditional emulsion, which is, thus far, the only economical form for this application, can plug pumps, nozzles, screens, or other apparatus, when the tedious clean out procedures necessary following application and if not done properly can lead to the lumping process described above. Microemulsions have been tested in this process and have been found to have the same problems because of the skinning described above.
Moreover, as noted, polyacrylamide requires surfactant and/or emulsions systems useful for admixing the polyacrylamide to form stable microemulsions. Specifically, a surfactant or emulsion system should be useful to effectively stabilize the aqueous discontinuous phase in oil to prevent phase separation and other like problems. The present invention provides emulsifier systems for stabilizing polyacrylamide-containing microemulsions.
Water repellant soils may cause serious issues when attempting to enhance the ability of plants to uptake water and other materials, such as fertilizers and/or pesticides. Specifically, water-repellant soils typically retard water infiltration into the soil matrix rendering the soil impervious to water penetration leading to underutilized application or misapplication moving away from the target area. Runoff of applications of fertilizers and/or pesticides, as well as soil erosion may result, especially during heavy rainfalls and/or irrigation conditions, causing fertilizers and/or pesticides to flow into water systems, such as reservoirs, lakes and rivers. Surfactants may be utilized to allow water infiltration of water-repellant soils, but many surfactants tend to burn plants or cause other like damage to plants, such as agricultural products, growing in the water-repellant soils.
Another form of water-repellant soil is so-called “crusted” soils, such as soils that have high amounts of organic matter built up on or near the surface of the soils. The crust may act as a barrier for the penetration of water, especially aqueous systems, which would be useful for providing water, fertilizers and/or pesticides to the root systems of plants.
Because it may be difficult to deliver pesticides and/or fertilizers to roots through water-repellant soils, known methods of delivering materials, such as pesticides specifically, may include boring into trees or other vegetation to deliver the useful materials. Of course, boring causes damage to trees and other vegetation, thereby weakening the trees or other vegetation.
It is generally known that ethylene oxide/propylene oxide (EO/PO) block copolymer has useful properties for wetting soils, for example, especially when used in agricultural, turf, ornamental or other compositions, especially on water repellant soils. However, EO/PO is notoriously difficult to mix into oil-containing systems.
EO/PO block copolymer has heretofore not been combined with W/O emulsions of polyacrylamide to obtain the useful properties of both the polyacrylamide (water retention) and the EO/PO block copolymers (water penetration). This is so because one would not expect EO/PO block copolymer to be useful in mixing in such a system because of its inherent incompatibility with oils. An EO/PO block copolymer is a long chain polymer made with ethylene oxide and propylene oxide portions. Thus, one would not expect EO/PO to mix well in W/O emulsions, especially due to the fact that EO/PO block copolymer hydrophobic portion, the PO block portion, is not a good lipophile or oil-loving molecule. Since, typically, emulsifiers (molecules that contain both water-loving and oil-loving portions) must contain a strong lipophile to be an effective ingredient in an emulsions or microemulsion, one of ordinary skill in the art would not look to include EO/PO block copolymer in a W/O emulsion, especially in combination with the polyacrylamide.